262 GEOLOGY. 



supposed that the crystalline rocks were the- first form- 

 ed in all cases, which were therefore called primary, or 

 primitive, the secondary being those which were be- 

 tween the primary and tertiary in relative age. But 

 when it was found that the so-called primary rocks were 

 formed in various ages, some of them quite recently, this 

 classification was given up ; but the name tertiary has 

 been retained, by common consent, as a matter of con- 

 venience. Indeed, the three terms are used still to some 

 extent, as mentioned in 261, primary having, however, 

 a different meaning attached to it from what it had for- 

 merly, it being applied to the palaeozoic rocks instead of 

 those which are originally crystalline. Lyell has divided 

 the strata of the Tertiary age into three series, according 

 to the percentage of shells found that are the same with 

 those that exist now. 1. Eocene, the term being derived 

 from two Greek words, eos, dawn, and kainos, recent. 

 When the name was adopted it was supposed that from 

 five to ten per cent, of the species found in the strata 

 were identical with species found at the present time, 

 and therefore the period in which these strata were 

 formed was very properly considered as the dawn of the 

 present life of the earth. But Dana states that it has 

 been discovered that the species of this period are all 

 extinct. 2. Miocene, the term coming from meion, less, 

 and kainos. Here from ten to forty per cent, of the spe- 

 cies found are living species at the present time. 3. Pli- 

 ocene, from pleion, more, and kainos. Here the percent- 

 age of living species is greater than in the miocene, it 

 being from fifty to ninety per cent. These three series 

 are often called ancient tertiary, middle tertiary, and 

 modern tertiary. 



369. Tertiary Deposits. In the previous ages the stra- 

 ta of rocks were mostly marine that is, deposited from 

 the waters of the sea as sediment, or laid down by coral 

 and other animals that lived in the sea. But now many 

 of the strata were either lacustrine that is, deposited 



